We use Magnetosphere Multiscale (MMS) mission data to investigate a small
number of magnetosheath jets, which are localized and transient increases in
dynamic pressure, typically due to a combined increase in plasma velocity and
density. For two approximately hour-long intervals in November, 2015 we found
six jets, which are of two distinct types. (a) Two of the jets are associated
with the magnetic field discontinuities at the boundary between the
quasi-parallel and quasi-perpendicular magnetosheath. Straddling the
boundary, the leading part of these jets contains an ion population similar
to the quasi-parallel magnetosheath, while the trailing part contains ion
populations similar to the quasi-perpendicular magnetosheath. Both
populations are, however, cooler than the surrounding ion populations. These
two jets also have clear increases in plasma density and magnetic field
strength, correlated with a velocity increase. (b) Three of the jets are
found embedded within the quasi-parallel magnetosheath. They contain ion
populations similar to the surrounding quasi-parallel magnetosheath, but with
a lower temperature. Out of these three jets, two have a simple structure. For these two jets, the increases in density and magnetic field strength are correlated with the dynamic pressure increases. The other jet has a more complicated structure, and no clear correlations between density, magnetic field strength and dynamic pressure. This jet has likely interacted with the
magnetosphere, and contains ions similar to the jets inside the
quasi-parallel magnetosheath, but shows signs of adiabatic heating. All jets
are associated with emissions of whistler, lower hybrid, and broadband
electrostatic waves, as well as approximately 10 s period electromagnetic
waves with a compressional component. The latter have a Poynting flux of up
to 40
Small-scale, transient increases in magnetosheath dynamic pressure have
recently attracted increased attention
Magnetosheath jets can be an important factor in the solar wind–magnetosphere
interaction, since their increased momentum may cause the magnetosphere to
deform locally when it is impacted by the jets
Several different definitions of magnetosheath jets have been used in earlier
studies. Either the dynamic pressure is required to be larger than some value
related to the background magnetosheath, often defined by a running average
The properties of magnetosheath jets have been studied both on a case study
basis and statistically, and some of their properties have begun to be
determined. Their scale sizes are of the order of 1
Magnetosheath jets and plasmoids may be associated with plasma wave emission,
similar to what has been found for fast flows in the magnetotail
In two large statistical investigations
Magnetosheath jets are most commonly observed downstream of the
quasi-parallel bow shock, and the upstream IMF is steady for a large majority
of the jets
Several theories for the formation of magnetosheath jets have been
considered, but presently interest is focussed on two main mechanisms.
Based on the observation that some jets are associated with IMF
discontinuities,
The second main theory was suggested by
While jets have been studied for close to 20 years, many of their
detailed properties are not well established, and are based on relatively few
case studies. We will here revisit some of these properties by using data
from the state-of-the-art instruments of the Magnetospheric Multiscale (MMS)
mission. Recently
We will present the data in Sects. 2 and 3. The interpretation and discussion will take place in Sect. 4. Section 5 contains a summary and conclusions.
The four spacecraft of the MMS mission
The MMS spacecraft are equipped with an extensive suite of state-of-the-art instruments
Due to the small separation of the MMS spacecraft compared to the phenomena studied here, we mainly use data from MMS1, except for the calculation of the current density and thermal pressure gradients, where multi-spacecraft methods are used. All vector quantities and positions are given in the geocentric solar ecliptic (GSE) coordinate system.
We will investigate two approximately hour-long intervals in November 2015, each containing three magnetosheath jets.
Figure
Back-tracking of jets no. 1 and 2 from their point of observations
in the GSE
In Fig
During the time interval shown in Fig. 1b, there are three distinct
increases in the dynamic pressure (marked 1–3), which were observed by MMS1
at a position of
Relative contribution of the density changes
Ion distribution functions in the planes formed by the direction of
the local magnetic field
Jets no. 1 and 2 have a clear increase in the
All three jets are associated with density enhancements of over 50 %, and
therefore also fulfill the plasmoid criterion used by
Figure
For jet no. 1 the ion distribution is very similar to the distribution for the
general quasi-parallel magnetosheath, in that it is quite isotropic (although
with a lower temperature, as can be verified by the temperatures in Fig.
For jet no. 2, the situation is more complicated. Being collocated with the
magnetic field discontinuity marking the transition from a quasi-parallel to
a quasi-perpendicular magnetosheath configuration, it contains two types of
ion populations. At the leading edge of the jet, a distribution function from
approximately 10:00:49 UTC is shown in Fig.
Finally, jet no. 3 is associated with an ion distribution with has a lower temperature than that of the surrounding quasi-perpendicular magnetosheath. It also has a clear temperature anisotropy, which, however, is smaller than that of the surrounding plasma.
Magnetic field configuration. From top to bottom: dynamic pressure, ion velocity, ratio of perpendicular velocity to absolute velocity, magnetic field, magnetic field smoothed with a 120 s window, magnitude of magnetic field, and magnitude of magnetic field smoothed with a 120 s window.
Recently both observations and simulations have shown that the magnetosheath
jets may be related to a local deformation or stretching of the magnetic
field
While the ion velocity is consistently perpendicular to
Jet no. 2 is clearly associated with a magnetic field discontinuity apparent in
the rapid change of
Jets no. 2 and 3 show rather clear increases in
The quasi-parallel magnetosheath typically exhibits large-amplitude,
low-frequency waves, that may originate either from the upstream solar wind
and foreshock, or from local instabilities
High- and medium-frequency wave activity. From top to bottom: dynamic pressure, on-board omnidirectional electric and magnetic field power spectral density (with the electron cyclotron frequency indicated by the white line), omnidirectional electric and magnetic field constructed from the survey and slow survey data, respectively. For the electric field spectrogram, the local lower-hybrid frequency is indicated by the black line.
Figure
Figure
Low-frequency wave activity. From top to bottom: dynamic pressure, absolute value of magnetic field, and magnetic and electric field components. For all panels (except the top one) the DC component has been removed by subtracting a 25 s running average.
Figure
Inside the quasi-parallel magnetosheath, at approximately 09:37–10:00 UTC, there is a general weak increase in the fluctuations in the magnetic field, with a compressional component. The fluctuation level is markedly increased inside jet no. 1 and at the leading edge of jet no. 2. Clear increases can be seen also at around 09:55 UTC, where a small increase in the dynamic pressure is observed, and collocated with jet no. 3, although this is outside of the general quasi-parallel magnetosheath region. The electric field shows a similar behaviour.
Detailed view of low-frequency wave activity for jets no. 1
A more detailed look at jet no. 1 in Fig.
In Fig.
Forces on the jet plasma. From top to bottom: dynamic pressure,
magnetic field, ion spectrogram, total thermal pressure for satellites 1–4,
negative of the gradient of the total pressure, and the magnetic force
Not much is known about the evolution of magnetosheath jets, as they move
downstream from the bow shock. The plasma inside the jets may be either
accelerated or slowed down by forces acting on it. The main candidate forces would
be either the
Overview of MMS1 measurements in the magnetosheath on 23 November 2015.
From top to bottom are shown dynamic pressure, magnetic field
Before we begin to interpret the above observations and compare them to earlier results and theories, we should ask how typical they are. This is a question that should and will be addressed by a statistical investigation. In the meantime, we will present observations from another magnetosheath jet event (in slightly less detail) to show that the present observations are mostly not unique to this particular case. We will do this in the following section.
Figure
Jets no. 5 and 6 have a very well-defined structure, with an isolated
appearance in panel (a), while jet no. 4 is more complicated, with appreciable
substructure. Jets no. 5 and 6 also have a clear density increase, collocated
with the
Similarly to jet no. 2 of the first event, jet no. 6, which is located at the
trailing edge of the quasi-parallel magnetosheath region, is collocated with
the boundary between the quasi-parallel and quasi-perpendicular magnetosheath,
and is associated with a pronounced magnetic field discontinuity. It has a
larger maximum in
Jets no. 4 and 5, however, have a low ion temperature anisotropy,
comparable to the rest of the quasi-parallel magnetosheath. Also, for these
jets, inspection of distribution functions show similar isotropic ion
distributions as for jets no. 1 and 2. Jet no. 5 has an ion temperature which
is clearly anti-correlated with the dynamic pressure. For jet no. 4 the
temperature is generally depressed, but it is difficult to see a detailed
anti-correlation with
The forces acting on the plasma are shown in panels (h) and (i) and exhibit a similar behaviour to the first event. The pressure gradient force is acting to break the magnetosheath plasma as it moves towards the magnetopause, with no clear correlation with the jets. There is some small-scale variation in the magnetic force within the quasi-parallel magnetosheath region; however, no clear signature related to the jets can be discerned.
Magnetic configuration for jets on 23 November 2015, in the same format as
Fig.
In Fig.
Jet no. 6 is clearly associated with a magnetic field discontinuity (Fig.
Both jet no. 5 and 6 are associated with a relatively clear increase in the
magnetic field magnitude (Fig.
From Fig.
In Fig.
Some of the properties of jets no. 1–6 are summarized in Table 1.
From Figs.
Jets no. 1, 4, and 5, however, are embedded within the quasi-parallel magnetosheath and are not clearly associated with any magnetic field discontinuity. They contain only one type of plasma population, with an isotropic ion temperature, similar to that of the surrounding quasi-parallel magnetosheath.
Jet no. 3, which has likely interacted with the magnetopause, has similar properties to that of jets no. 1, 4, and 5, but has a larger ion temperature anisotropy. We will briefly discuss this jet further below.
The different properties of the two types of jets may point to different generation mechanisms. We will first discuss the jets associated with a magnetic field discontinuity.
Summary of properties of the jets investigated.
Jets associated with magnetic field discontinuities have been suggested by
There are, however, a few problems with applying the results of
Furthermore, the pressure pulses of
This scenario could be tested by observations closer to the bow shock than
those reported here. It also predicts that large increases in
A remaining difficulty with this scenario is that jets no. 2 and 6 are
associated with a clear decrease in both the perpendicular and parallel ion
temperature. It is difficult to explain this other than as a signature of
solar wind or foreshock plasma that is less processed by passing through the bow
shock than the rest of the magnetosheath. This is one of the main predictions
of the mechanism proposed by
The above scenario therefore needs to include an interaction with the bow
shock according to the Hietala model, where the jet will pass the bow shock
at local indentations, allowing the plasma to pass with much less
deceleration and heating than the surrounding solar wind plasma. This is an
alternative way of imposing a jet scale size perpendicular to the flow (see
Fig.
Additional acceleration of the jet plasma could also conceivably result if
the primed, stretched magnetic field lines, produced by the interaction of the
discontinuity with the bow shock, could be turned around and be more aligned
with the Sun–Earth line. One way this could happen is if part of the
stretched field structure passes through a bow shock corrugation, and if the
original normal to the discontinuity already has some angle to the
Six different scenarios for formation of the jets in this paper. FW
Turning to jets no. 1, 4, and 5, none of these are associated with a clear
magnetic field discontinuity, or with a clear particle boundary. In that
sense they are consistent with the model proposed by
Jet no. 5 has a very well-defined shape with clear correlations between
An alternative scenario producing an increased plasma density and magnetic
field strength is the focusing of fast plasma associated with a concave bow
shock corrugation (Fig.
We also note that, in contrast, a convex corrugation would produce diverging
plasma flows (Fig.
Jets no. 1, 4, and 5 all contain a plasma with isotropic ion temperatures,
similar to the surrounding quasi-parallel magnetosheath plasma, but cooler.
This is consistent with the results reported by
Jet no. 3, however, contains a cool plasma with a pronounced temperature
anisotropy. The anisotropy is due to an increased perpendicular temperature,
although not as high as in the surrounding quasi-perpendicular magnetosheath.
It is likely that this perpendicular heating is associated with an
interaction of the jet with the magnetopause. The anisotropic temperature may
be due to a compression of the jet plasma, with associated adiabatic heating
For both dates examined, we observe several kinds of wave emissions. There
are increased spectral densities at and below the local lower hybrid
frequency, mainly in the electric field. The emissions are seen both inside
and outside of the jets, but with a generally increased amplitude inside of
them, in particular for jets no. 3 and 5, and the leading parts of jets no. 2
and 6. This is consistent with the reports of lower-hybrid wave emissions
by
Likewise, emissions at a few tens of percent of the local electron
gyrofrequency, for both
The generation of these waves will not be studied further here, but will be
the subject of future work. However, we note that
A new result here, compared to the study by
A further new result is the existence of low-frequency, quasi-periodic
electromagnetic oscillations, with higher amplitudes within the jets than in
the surrounding quasi-parallel magnetosheath. Here we have direct
observations of Poynting flux. The highest values of these fluxes are of the
order of 50–60
ULF wave activity in the magnetosheath is often attributed to convection of
waves generated in the foreshock or at the bow shock
Diffuse ions are the hot, isotropic part of the suprathermal ions of the
foreshock and quasi-parallel magnetosheath
If the diffuse ions are responsible for the generation of the low-frequency waves in the jets, the waves will not dissipate the kinetic energy of the jets, and may not influence their propagation. However, they may play an important role in thermalizing the magnetosheath plasma associated with jets.
We have already discussed the fact that no clear signature in the magnetic
force could be found for the jets associated with the magnetic field
discontinuities. No such signatures can be seen for any of the other jets
either. The same is true for the thermal pressure gradients. We conclude that
for the jets studied here, no appreciable acceleration or braking due to such
forces take place close to the magnetopause. This is in contrast to forces
acting on fast flows (bursty bulk flows) in the magnetotail, where
We have used MMS data to carry out a detailed analysis of the large- and
meso-scale properties of magnetosheath jets with a modern suite of plasma
instruments, based on the current understanding of such jets. We have
studied six different magnetosheath jets during two time periods in November,
2015, and have made the following conclusions:
We have found that there are at least two distinct types of jets, with some clearly different properties.
Two of the jets are associated with a magnetic field discontinuity and straddle the boundary between
the quasi-parallel and quasi-perpendicular magnetosheath. The different parts of the jets, therefore, contain
different plasma populations: one isotropic ion population and another with a larger ion temperature anisotropy.
Both ion populations are, however, cooler than the surrounding magnetosheath plasma. This type of jet had a larger
maximal dynamic pressure than the second type, and although it may be more rare than the second one, it may therefore
have a greater impact on the magnetopause interaction. The dynamic pressure is correlated with both velocity and
density, as well as with magnetic field strength. The second type is found embedded within the quasi-parallel magnetosheath, has no clear association
with a magnetic field discontinuity, and contains a plasma with an isotropic ion distribution similar to the
surrounding plasma but considerably cooler. This type of jet can either have a simple, well-defined structure,
in which case it has a clear correlation with increases in plasma density and magnetic field strength, or a
more complicated morphology, with a more unclear relation to density and magnetic field increases. One jet that contained a quasi-parallel magnetosheath ion population, and therefore probably was an example
of the second type, was found outside of the quasi-parallel magnetosheath. It is likely to have interacted
with the magnetopause, and showed a weak ion anisotropy, possibly as the result of adiabatic heating. All the jets studied here were associated with medium- and high-frequency wave emissions in the lower hybrid
and whistler frequency ranges, as well as broadband electrostatic waves. None of these wave emissions were found
to be energetically important, and they are therefore not likely to affect the evolution of the jets. The isotropic ion populations within the jets were found to be associated with low-frequency electromagnetic
waves with a compressional component. These waves had a Poynting flux of up to 40 A majority of the jets showed no clear field-aligned flow of the type reported by No clear signatures of magnetic or thermal pressure gradient forces were seen in association with the
magnetosheath jets investigated here.
We have suggested that the two different types of jets are associated with
different generation mechanisms, and have discussed different scenarios for
their creation. The different nature of these types of jets suggests that
future simulations and modelling of jets should address both types. In
particular, 3-dimensional simulations may reveal new insights into the jet
generation mechanisms for both types of jets. Likewise, separating the two
different types in future statistical studies may help to reveal further
differences in their properties, such as scale sizes, occurrence,
geoeffectiveness
The wave emissions associated with the jets will be the subject of future study, in particular the role of the low-frequency waves, both in terms of affecting the evolution of the jets, and influence on the magnetopause. Also, the relation between the flow velocity of the jets and the magnetic field configuration, where no consistent picture has emerged, will have to be the subject of further study. This may be of importance for the way magnetic forces may influence the dynamics of the jets in the magnetosheath, and the interaction of the jets with the magnetopause.
The MMS data used in this study are available at the project home
page
The authors declare that they have no conflict of interest.
We acknowledge valuable discussions within the International Space Science Institute (ISSI), Bern, team 350 “Jets downstream of collisionless shocks”, led by Ferdinand Plaschke and Heli Hietala. We acknowledge NASA contract NAS5-02099 and Vassilis Angelopoulos for use of data from the THEMIS Mission. Specifically we thank Karl-Heinz Glassmeier, Hans-Ulrich Auster and Wolfgang Baumjohann for the use of FGM data provided under the lead of the Technical University of Braunschweig and with financial support through the German Ministry for Economy and Technology and the German Center for Aviation and Space (DLR) under contract 50 OC 0302. The work of Heli Hietala is supported by NASA grant NNX17AI45G and contract NAS5-02099. The topical editor, Georgios Balasis, thanks two anonymous referees for help in evaluating this paper.